Manufacture of printing plates



March 4, 194:1

M. SQRK IN MANUFACTURE OF PRINTING PLATE Filed Oct. 4, 1.940 5 Sheets-Sheet l VINVENTOR I ORNEY March 4, 1941. M. SORKIN MANUFACTURE OF PRINTING PLATE .3 Sheets-Sheet 2 \NVENTOR A O NEY Filed Oct. 4, 1940 March 4, 1941. so 2234.133

mmumcwum: or PRINTING PLATE Filedoct. 4, 1940 s Sheets-Sheet 3 Patented Mar. 4, 1941 UNITED STATES OFFICE Morris Sorkin, New York, N. Y., assignor to Enterohemical Corporation, New York, N. K, a. corporation of Ohio Application October 4, 1940, Serial No. 359,644

4 Claims.

This invention relates to contrast images for making printing plates, and to methods of making such contrast images.

The invention aims to providea printing plate 5 from which one may make prints containing printed and unprinted areas so proportioned and arranged as to give to the eye the effect of the variations in tone of an original subject or a continuous-tone photographic image thereof. Such a print I shall term a contrast image" of the original to indicate that it contains only two contrasting tones, variations in the areas of which are utilized to simulate continuous variation in tone.

In order that a contrast image may give to the eye the efiect of the tone variations of a continuous-tone image, the black and the white areas are so arranged and proportioned that the ratio of black area to White area corresponds to the tone of the original in each part of the contrast image. The area of the contrast image thus consists of a large number of small elemental areas, each of which includes a black area and a white area so proportioned that the ratio of the size of the black area to the size of the whole elemental area corresponds (more or less closely according to the method used) to the total value of the area of the continuous-tone original corresponding to this elemental area of the contrast 30 image. In order to save repetition and to simplify the terminology used in this application, I shall refer to such an elemental area in which the ratio of black area to the total area corresponds to the value of the tone of the corre- 35 sponding area of the continuous-tone original as an element of the area of the contrast image.

My invention relates to the type of contrast image which differs from the ordinary half-tone image in that the elements of the image corre- 40 sponding to different tones are of different sizes. In such contrast images, which are ordinarily prepared by photo-electric scanning, the larger elements, for example, those representing the lighter tones, are usually much greater in length 45 than in width. It has been found that objectionable patterns occur in certain more or less constant tone areas of such images where the elements are long. These are usually lines extending transversely to the long dimension of the 50 elemental areas.

I have discovered that it is possible to eliminate such objectionable lines by making, between adjacent rows of elements, a slight change in the length of the element representing each 55 tone. My invention based on this discovery pr0- vides a contrast image free from objectionable pattern effects and consisting of rows of elements of difierent lengths representing different tones, the lengths of the elements representing each tone being different in adjacent rows.

My invention also includes a photo-electric scanning apparatus for producing such a contrast image, which apparatus is claimed in my copending application Serial No. 172,355, filed November 2, 1937, on which Patent No. 2,222,991 10 has been issued on November 26, 1940. In the scanning apparatus heretofore used for making contrast images having elements of .difierent lengths, it has been customary to control the length of either the black or the white part of 15 each element, or both the black and the white parts, by the time required for the charging or discharging of an electric condenser by a current controlled by a photo-electric cell scanning the original to be reproduced. In accordance 20 with my invention, such apparatus is modified by introducing mechanism operative at each scanning line to vary the time cycle of the charging and discharging, as, for example, by making ;a change in the capacity of the condenser.

I will explain my invention in detail in connection with the annexed drawings, in which:

Fig. 1 is a greatly enlarged view of part of the contrast image embodying my invention;

Fig. 2 is a similar view of a somewhat more desirableform of contrast image embodying my invention;

Fig. 3 is a similar view showing the objectionable pattern efiect which has occurred in previous contrast images;

Fig. 4 is a diagram of a photo-electric scanning apparatus of the drum type embodying my invention;

Fig. 5 is a vertical section on an enlarged scale taken on the line 5-5 of Fig. 4; Figs. 6 and 7 are diagrams showing a photoelectric scanning apparatus of the table type embodying my invention, omitting the parts of the electric circuit which are the same as in the drum type scanning apparatus of Fig. 4; and 45 Fig. 8 is an edge view of the ratchet and disc shown in Figs. 6 and 'l.

Fig. 1 shows a light, constant-tone area of a contrast image embodying my invention. This area consists of a series of rows of long narrow elements, each of which contains a large white area. and a small black area. The ratio of the black area of each element to the total area of the element corresponds to the tone represented. Although all the elements represent the same tone, the length of the elements in the alternate rows A is greater than the length of the elements in the other alternate rows B, so that definite objectionable lines cannot be produced as in the former type of contrast image illustrated in Fig. 3. Both the long white areas and the short black areas are proportionately reduced in length in the alternate rows, so that, notwithstanding the difference in the length of the elements in the alternate rows, the black and white areas of each element correctly represent an area or the original which is of uniform tone.

Fig. 2 shows a light, constant-tone area of a contrast image embodying my invention in a still more desirable form. In this case, the elements representing the tone are of three different lengths. Those in the rows A are the longest, those in the rows C the shortest, and those in the rows B intermediate between the two. This still more effectively eliminates pattern eiTects.

A scanning apparatus for making the type of contrast image shown in'Fig. 1 is illustrated in Figs, 4 and 5. It includes an ordinary electromechanical scanning apparatus having a, transparent picture drum I0 and a recording drum I I, wwhich are given identical rotational and longitudinal movements through a driving mechanism l2. A constant intensity lamp' l3 regulated by resistance 41 and provided with a suitable lens system H is mounted to scan a continuous-tone transparent original or a continuous-tone transparent photograph of an original mounted on the drum ID. The light from the scanning lamp [3, as modified by passing through the transparent original, is received by a photo-electric cell l5.

A recording lamp l6 illuminating'a shield i1 containing a long narrow slit or aperture I8 is provided with a lens system l9 which focuses an image of the slit l8 on a recording film placed on the drum H. In the form illustrated, the shield" containing the slit I8 is the anode of the lamp, but this is not essential as the shield containing the slit maybe separate from the lamp. The lamp is a gas-discharge lamp having no afterglow. The length of the image of the slit I8 is equal to the distance between the turns of the screw 20 of the driving mechanism,

so that, as the mechanism operates, the image of the slit scans the entire area of the recording film without any overlap. The length of the image of the slit i8 on the recording film determines the width of the elemental areas of the contrast image produced on the recording film. The lengths of the elemental areas are determined by the time length of the cycle of the recording lamp l8; and the proportion of black and white which each elemental area contains is determined by the time during which the recording lamp i6 is lighted and the time period during which it is extinguished during one cycle. These time periods are controlled by the light passing through the original to the photo-electric cell l5 by means of interconnecting electric circuits between the photo-electric cell If: and the recording lamp IS The interconnected electric circuits include a photo-cell circuit which charges a condenser 35, a constant current circuit which discharges the condenser 35, a trigger circuit actuated by the voltage across the condenser 35, and a control circuit for the recording lamp l6 actuated by the trigger circuit. The circuits are so interconnected that the condenser 35 is alterpotential is built up on this plate when the current flows in the photo-cell circuit. The current in this circuit and the rate of charge of the condenser are proportional to the amount of light reaching the photo-cell.

The discharging circuit is connected to the plate 35a of the condenser and contains a source of electro-motive force 28 opposed to the source of electro-motive force 21 in the photo-cell circuit. The discharging circuit also contains means for maintaining the flow of current constant and means for stopping and starting the flow of current. These means consist of a vacuum tube 2! whose plate circuit constitutes a part of the discharging circuit. The tube 2! contains a grid 2la maintained at a constant potential and holding the flow of current in the plate circuit at a constant value greater than the greatest current in the photo-cell circuit. While greater than the current in the photo-cell circuit, the discharging current should be of the same order as that in the photo-cell circuit which requires that the tube 2i be operated at very low voltage. The strength of the discharging current may be regulated by a potentiometer 53 connected to the grid 2m and spanning the source of electro-motive force 28. The discharging current is most desirably made equal to the sum of the maximum and minimum values of the charging current, that is, the val ues of the current in the photo-cell circuit when the maximum and minimum amounts of light are received on the photo-cell. The tube 2| also contains a control grid Zlb which permits the plate current to flow when it is neutral with respect to the cathode of the tube and cuts off the flow of current when it is at a certain negative bias with respect to the cathode of the tube. This grid is connected by a variable tap 49 to the resistor 38 of the trigger circuit hereinafter described. When the current is flowing in the discharging circuit, it reduces the positive charge on the plate 350, because the current in this circuit is greater than the current in the photo-cell by which it is opposed.

The trigger circuit is actuated by the voltage across the condenser 35 in such manner as to cause a drop in potential in the resistor 38 when the condenser voltage has fallen below a predetermined value (which, for convenience, I shall term the low trigger value) and to eliminate the drop in potential in the resistor 38 when the condenser voltage rises above a predetermined value (the high trigger value"). The trigger circuit includes a vacuum tube 22 whose grid is connected to the plate 35a of the condenser and whose plate circuit is connected to cause a flow of current through a resistor 31. The resistor 31 is connected through an oscillator and detector (enclosed in dotted lines in Fig. 4) to the resistor 38. The operation of this detail) is such that it causes a flow of uni-directional current through the resistor 38 when the 7s flow of current through the resistor 31 is such as occurs in the plate circuit of the tube 22 when its grid falls below the low trigger value, and cuts off this current through the resistor 38 when the potential of the grid of the tube 22 rises above the high trigger value.

The control circuit for the recording lamp I6 is connected to the resistor 38 of the trigger circuit, and-serves to keep the recording lamp l6 lighted only when there is a drop in potential in the resistor 38 or only when there is no drop in potential across this resistor, according to the setting of a reversing switch 48. The control circuit includes a source of electro-motive force 34, the recording lamp l6, and the plate circuit of a tube 26. Current flows. in this circuit only when the grid of the tube 26 is neutral. When the reversing switchis in the position marked Neg, this occ rs only when there is no drop across the resist r 38, which in turn occurs when the oscillator and detector are in the static con dition. With the switch in the same position, and the oscillator and detector in the dynamic condition, the drop across resistor 38 provides sufficient negative bias to the tube 26 to prevent flow of any plate current therein. Therefore, the recording lamp i6 is lighted when the oscillator and detector are in the static condition, which occurs during that part of thecycle' when the discharging tube 2| is passing plate current. If the area being scanned at this moment is highly transparent, the charging current due to the photo-cell will-be large. Therefore, as this opposes the discharging of the condenser, this part of the cycle will be long and will produce a long black dot. Similarly, the recording lamp is oil when the oscillator and detector are in the dynamic condition, which occurs during that part of the cycle when the condenser. 35 is being charged up to the high trigger value" by the charging current. If the area being scanned is transparent, this charging of the condenser will take place rapidly and a small white space will be formed. This results,-therefore, in making a negative contrast imageof a positive subject on the drum Ill. When the switch 40 is thrown to the position marked Pos. in Fig. 4, the grid circuit of the tube 26 is connected across a resistor 39in the plate circuit of a tube 25 whose grid is then connected to the low' end of the resistor 38. In this case, the current in the plate circuit of the tube 26 and the recording lamp it; flows only when there is no current in the plate circuit of the tube 25, that when there is a potential drop in the resistor 38. In this case, a positive contrast image is made.

The operation of the interconnected circuits which have been described is cyclic and may conveniently be described by beginning at the end of the discharge of the condenser 35. At this time, there is a potential drop in the resistor 38, as the condenser voltage is below the trigger values; if the reversing switch is in the positive position, the recording lamp is on and no current is flowing in the discharge circuit which includes the plate circuit of the tube 2|. The condenser, therefore, becomes charged by the current flowing through the photo-cell circuit, and the charge continues to increase until-it reaches the high trigger value. At this point, the trigger circuit operates to stop the flow of current in the resistor 38 and eliminating the potential drop in this resistor. The elimination of the potential drop in the resistor 38 eliminates the negative bias on the control grid 2|b of'the tube 2| and starts the flow of current in the discharge circuit. It also operates the control circuit to extinguish the recording lamp l6. Following these two efiects of the elimination of the voltage drop.

in the resistor 38, which take place simultaneously, the condenser voltage decreases, since the current in the discharging circuit is greater than the current in the photo-electric circuit which is opposing it. The discharge of the condenser continues until its voltage reaches the low trigger value. The trigger circuit then starts a flow of current through the resistor 38, causing a potential drop in this resistor. The potential drop in the resistor 38 produces a negative bias on the control grid 2|b of the tube 2| suflicient to, stop the flow of current in the discharge circuit. It also lights the recording light l6. This completes the cycle and the charging of the con.-

. denser by the current in the photo-cell circuit begins again. A milliammeter 46 may be connected in the recording lamp circuit to indicate the proportion of black in the contrast image and may beadjusted by a variable shunt 48 to read I06 when the recording light is operated without interruption, a condition which may be attained by closing a. switch 5| to maintain a constant potential on the grid of tube 26. An ampere hour meter 60, connected in the recording lamp circuit, may be used for indicating the total black area in the contrast image.

In the cyclic operation which has been described, the rate at which the condenser 35 is frequency of the cyclic operation depends upon the capacity and other constants of the cyclic circuit and also upon the strength of the photo-' cell current fed to the circuit. At any fixed value of the electrical constants of the cyclic circuit, which determine the so-called time constant of the circuit, the rate at which the condenser 35 is charged is proportional to the amount of light reaching the photo-cell, as this determines the strength of the current in the photo-cell circuit; and the rate at which the condenser is discharged is an inverse function of the amount of light reaching the photo-cell, since the photo-cell current opposes the constant current in the discharging circuit. The time intervals of the charging and discharging" of the condenser depend upon the charging and discharging rates and determine the lengths of the blackand white portions of each element of the contrast image.

This results in making the ratio of black area to total area of each element equal to the value of the tone of the corresponding portion of the original.

When a dark tone area of the original is between the scanning lamp and the photo-cell, the photo-cell current is weak, so that the charging of the condenser to the'high trigger value takes a long period of time making a long black area on the contrast image, and the discharge of the condenser is rapid making a short white area. Because of the slowness of the charging, the whole cycle is long, and consequently the elemental area is long.

When a light tone area of the original lies between the scanning lamp and the photo-cell, the photo-cell current is large, chargin th ndenser rapidly and opposing the discharging current so that the discharge is slow. This also results in a long cycle producing a long elemental area which, in this case, contains a long white area and a short'black area.

. When the middle tone of the original is be.

ture scanned arm 15.

tween the scanning lamp and the photo-cell, the

in size. l My invention comprises means which may be combined with this circuit for causing periodic; changes in the sizes of the elements without varying the relation between the sizes of'the elements representing different tones and without varying the proportion of black tov white in each element.,. It accomplishes this result by causing, aproportional variation in the times required for charging and discharging the condenser for each value of the photo-cell current.

-My invention may most simply be incorporated in a scanning device of the type which has been described-that is to say, in one which has an electric circuit having a cyclic operationby introducing means operated periodically, most desirably at the end of each scanning line, to modify the electric circuit by altering one of its constants. Such alteration makes, a

change in the maximum frequency of the cyclic operationof the circuit which, as stated above, occurs at a middle tone of the original. Equal percentage ,changes are also caused in the lower frequencies of the cyclic operation of the circuit which, as notedabove, occur in light and dark toneareas of the original. Such alteration of the time constant of the-circuit may, for example be effected by changing the amount of the capacitycontained in the circuit.

The specific embodiment of my invention includes, an auxiliary condenser 35:: (Fig. 4) and aviswitch operated by the mechanism for moving thedrums-JIJ and H to connect this. condenser ih parallel with the condenser 35 of the scanning circuit during alternate revolutionsofthedrums.

.The auxiliary condenser 35:: is connectedv in parallelJwiththe condenseru35 bywires 10, 1|

- which are connected to brushes I3, 14 mounted on'ari'gid arm 15 secured to the -frame 6. These brushes are adapted tocontact-a semi-circular conductor. 16. (Fig.5) insetin a fibre'gear 11 which ismounted on. a shaft I8 carried by the I V The gear 11 meshes with a gear I9 secured to shaft 5 and may be twice the size of the latter gear, so that the ,brushes. complete thecircuit through condenser 35:0, and thus connect it in parallel with thecondenser 35, duringone out of each two revolutions of the drums l0 and II. This results in varying the length of the elements representing each .tone. in alternate lines of elements, without varying the proportion of black towhite in each *element, as shown in Fi 1,-

Another specific embodiment of. my invention is shown in Figs-6, 7 and 8 It is incorporated in a scanning device which .is like that which has-been described, except that the original picby the light [.3 and the contrast image produced bythe recording light it! are supported on a reciprocating table 80 instead of -upon rotating drums. As is usual in such arrangements, means (not shown) are provided tion, so that "forgiving the table so a slight lateral movement at the end of each oi? its strokes in eachdireceachstroke oi l'the 'table in,.ejach direction represents "one scanningline. J The reciprocatory movement'of the table is utilized 'to closefoneor the-ether of, 'two switche's' 8! at theQend of each stroke; The switches are connected to a solenoid 82 'oper'atinga pawl .83 which causes a' step-by 'sthj movement, jof a ratchet. 84 carryin an electflcjteilninal 85." At each scanning line, this terminal comesinto contact. with two of a number. of fixed terminals 86 on'a disc '81 'of insulating material adjacent to the ratchet. In. the 'formshown, thereare twelve pairs 86a, 86b, 860 of ffixed terminals as; sothat the terminal 85 on theratchet co ntacts with, the twelve pairs of fixed'fterminals successively. during twelve succ'eslsive scannin lines.

Theccontact, between the terminal on. the

ratchet and the fixed terminals is utilizedto make the efiective capacity of the condenser 35 of the scamling apparatus difierent in adja'cent scanning lines by connecting one or more auxiliary condensers in parallel with it." In the form showndiagrammatically in Fig. 7, there are two auxiliary condensers 35a; and 35g. A circuit provided by wires 88 serves to connect either or both of the" auxiliary condensers in parallel with the main condenser 35..v When the ratchet terminal as is in 'contact with the fixedterminals 86a, neither auxiliary condenser is connected. When it is in contact with. the fixed'term'inals 86b, the auxiliary condenser 35a: is connected in parallel with the main condensenand when it is in contact with theterminalsaiic, both auxiliary condensers are connected in parallel with the main condensen. ,This results in varying the length of the elements representing each tone in the manner shown in Fig. 2.

of scanning apparatus shown inRanger Patent My invention may ,also'be .applied to the type No. 1,848,839 or to the type of scanning apparatus shown in Ranger Patent No. 1,790,722, by utilizing apparatus such as that shown in Fig.15to connect auxiliary condensers in parallel with the condensers -used in the Ran'gencircuits during alternate'li'nes or turns of the scanning. The Ranger condensers which determine the lengthof the elements of the contrastimage and whose capacity is regularly varied infapplying myinvention to the Ranger circuits, are the condenser l3 of Figs. 1, 2, 3 of Patent No. 1,848,839, the

condensers l6 and I! jof Fig. lot Patent No; 1,790,722, and the condensers 42 and 43 of Fig. 2

of Patent No. 1,790,722. However, my'invention is especially applicableto, and in fact an improvement of, the scanning. apparatus disclosed in Hardy Patent No. 2,136,340.

. It will be appreciated thatit is not necessary tdchange the length of the elements at the end of eachsingle scanning line, although the best results are obtained by doing so. The change maybe made as often as is desired or necessary inorder to avoid pattern and lineeffects in the contrast image. The, operationof the oscillator and detector enclosed in dotted line in Fig, 4 is as follows:

,Whensufficient current flows through resistor.

31 the grid of tube .23., is held at a negative potential sufficient to prevent a current in1the plate circuit of this tube. ,When the current through resistor 31 decreases, thereis a corresponding increase in the potential of the grid of tube, 23. Whenpthisi increase in potential reaches a certain value, plate current will start to flow in this tube. This will occur when the voltage across condenser 35 has fallen to the low trigger value. The plate current flows through the primary of transformer 4|. This induces a voltage in the secondary of the transformer which causes the grid of tube 23 to become more positive by virtue of the potential induced on one plate of condenser 36. This results in a further increase in the plate current and a correspondingly further increase in the grid voltage until saturation conditions are attained. At this point, the current through the primary of transformer 4| becomes momentarily constant and the secondary voltage drops to zero. This causes a decrease in the potential of the grid of tube 23, which produces a corresponding reduction in the current through the primary of transformer 4|. This induces in the secondary of the transformer a voltage of the opposite sign and tends to make the grid of tube 23 even more negative. This process continues until the grid of tube 23 becomes so negative that the plate current ceases. Current then flows through the resistor 44 in such a direction as to again cause the potential of the grid of tube 23 to increase. These oscillations will now continue until the grid of tube 23 is biased to a certain negative voltage, dependent partly on the amount of feed-back from the secondary of the transformer 4| and partly on the value of resistor 44, when the oscillations will cease. This negative value is attained when the plate current in tube 22 increases, thereby increasing the voltage drop across resistor 31. This occurs when the voltage across condenser 35 has attained the high trigger value. Hence, the low trigger value is determined by the bias necessary to start oscillation in the oscillator and detector circuit while it is in a static condition, whereas the high trigger value is determined by the bias necessary to stop the oscillation when the oscillator and detector are in a dynamic condition.

Tube 24 is operated as a detector, its grid being biased normally slightly below the cut-off potential. During one-half of the oscillation period of tube 23, the voltage induced in the secondary of transformer 4| causes the grid of tube 24 to become positive allowing current to fiow in the plate circuit through the resistor 38. Since the oscillations of tube 23 are at a radio frequency, the oscillations of this tube result in a pulsating uni-directional current through resistor 38 which to all intent and purpose may be regarded and treated as a direct current. It is obvious to one skilled in the art that this oscillator and detector might be replaced by a direct current amplifier containing an odd number of stages.

What I claim is:

1. A contrast image consisting of rows of elements in each of which elements the ratio of black area to white area corresponds to the value of the tone of the corresponding area of an original, the lengths of the elements representing each tone being diiferent in adjacent rows.

2. A contrast image consisting of rows of elements in each of which elements the ratio of black area to white area corresponds to the value of the tone of the corresponding area of an original, the length of the elements representing each tone being difierent in adjacent rows, the proportion of black to white in the differentlength elements representing the same tone being equal.

3. A contrast image having a portion representing a uniform tone area of an original and consisting of rows of long, narrow elements, the elements being difierent in length in adjacent rows, and each element containing a black area and a white area whose ratio corresponds to the uniform tone of said area of the original.

4. A contrast image having a portion representing a uniform tone area of an original and consisting of rows of long, narrow elements, the elements being diflerent in length in adjacent rows, the shorter and longer elements each containing black and white areas bearing the same ratio to each other and corresponding to the tone of said uniform tone area 01' the original.

MORRIS SORKIN. 

